1. Field of the Invention
The present invention relates to a liquid crystal display device driven by a thin film transistor, and to a method of manufacturing the same.
2. Description of the Related Art
As to an image display device capable of reducing its size and weight as well as obtaining high definition, a market for a thin film transistor liquid crystal device (TFT-LCD) has been expanded. In recent years, along with increasing screen size, higher definition, and higher-speed driving of the TFT-LCD, a wiring material with lower resistivity is used. Moreover, a reduction in manufacturing cost has been strongly demanded for accommodating a lower price.
The wiring material has been shifted from chromium or molybdenum to aluminum along with a reduction in resistance, and currently, a molybdenum/aluminum/molybdenum laminated-film structure in which molybdenum or a molybdenum alloy is used for a cap layer or a barrier layer is a mainstream. However, a resistivity of pure aluminum is about 3 μΩcm, which imposes a limitation on a further reduction in resistivity. For example, making a thicker film made of aluminum involves problems such as a dramatic increase in frequency of occurrence of hillocks which cause defects, in addition to increased tact time in a film formation step.
In addition, as a barrier cap film, a film made of expensive molybdenum is necessary for an upper layer and a lower layer made of aluminum, which increases material costs. As a wiring material having a resistivity lower than that of aluminum, copper is conceivable. A thin film made of copper has a resistivity as low as about 2 μΩcm. However, there are several problems when copper is used for wiring. For example, copper has a low adhesion force with a base, or unnecessarily diffuses into silicon forming a thin film transistor, leading to deterioration of transistor characteristics.
In order to alleviate the above-mentioned defects, there is proposed, in Wen-Ching Tsai et al., pp. 1181-1184, Volume 37, SID International Symposium Digest of Technical Papers, 2006, a wiring structure in which a film made of a copper alloy is disposed between copper wiring and a base material to secure an adhesion force with a base. Further, our preliminary study reveals that an adhesion force with a base is secured when porous copper is disposed between the copper wiring and the base substrate.